The production of elemental silicon by the reduction of silicon tetrafluoride with sodium metal is known and is described in U.S. Pat. No. 4,442,082 and in Novel Duplex Vapor - Electrochemical Method for Silicon Solar Cells (31 March 1980), both of which are incorporated by reference. The reduction of the silicon tetrafluoride with sodium metal produces a reaction product that comprises silicon and sodium fluoride. This mixture is typically separated by leaching out the sodium fluoride using an acidic aqueous leaching liquid. The loss of silicon by oxidative degradation in this process has been addressed in application Ser. No. 758,595 filed as of even date herewith by Kenneth Sancier. That application is incorporated by reference and it describes the recovery of silicon with minimum losses of silicon due to oxidation by the addition of an aqueous solution of alkaline earth metal chloride which transforms the sodium fluoride into an insoluble fluoride that is readily separated from aqueous sodium chloride.
The process described in U.S. Pat. No. 4,442,082 is directed to the production of high purity silicon for use in solar cells and other applications requiring highly pure silicon. For this reason each reactant is a possible source of impurities which may contaminate the final product. It has been found that certain commercial sources of reagents may be unsuitable and it may be necessary to carry out procedures for analysis and/or purification of reactants before the reduction reaction is carried out. The present invention is directed to providing means for regeneration of sodium from the sodium chloride obtained in the fluoride precipitation of the sodium fluoride obtained from the reduction of the silicon tetrafluoride with sodium. Certain commercial sources of sodium contain traces of elements such as boron which exert a deleterious effect on solar grade silicon. In addition the reactive nature of sodium requires that it be shipped under a layer of hydrocarbon such as kerosene to prevent it from reacting with the atmosphere. The presence of the hydrocarbon is a source of carbon contamination in the final product.
It should be noted that the in situ regeneration of sodium from sodium fluoride is technically feasible but commercially unattractive because of difficulties in the operation of fluoride cells. The present invention requires that the sodium fluoride produced in the reduction of silicon tetrafluoride be transformed to sodium chloride. The reduction of the sodium chloride generated in this reaction and the recycle of this regenerated sodium in a closed system will avoid the necessity of obtaining pure grades of sodium metal or to refine certain commercial grades of the sodium metal prior to use. In addition the recycling of the sodium metal from the sodium chloride that is generated from the reaction of an alkaline earth metal chloride and sodium fluoride will improve the overall economics of the process.
Accordingly it is a primary object of this invention to provide a method for the regeneration and recycle of sodium metal in a process which utilizes sodium metal for the reduction of silicon tetrafluoride, titanium tetrafluoride, zirconium tetrafluoride, and uranium tetrafluoride or hexafluoride.
It is also an object of this invention to provide a closed loop process which eliminates the need to purify or assay sodium obtained from an exogenous source.